The use of two-way, or returnable mailing envelopes has been recognized as facilitating the transaction of business, for example, in the sending out of monthly statements by business firms where the customer is expected to return a portion of the statement with a remittance to expedite proper crediting of payments. Such return envelopes commonly utilized an extended seal flap having an upper portion adapted to be severed from the connection with the remainder of the seal flap, the remainder having seal gum or adhesive thereon for resealing the envelope. By way of example, U.S. Pat. No. 3,874,582 discloses such an arrangement wherein the upper portions of each of the side flaps together with other portions of the envelope, including the original seal flap, is removed allowing a top portion, formally integral with the seal flap, to be utilized for sealing in preparation for a second mailing. Also note U.S. Pat. No. 3,982,689 showing a returnable mailing envelope having a compound seal flap including an upper portion separated from a lower portion by a tear line and seal gum or adhesive strips extending along each portion.
Prior art returnable envelopes, however, tended to be complex, either in structure or in manufacture, necessitating a relatively high cost to the user.